Electronic auxiliary contact for a contactor

ABSTRACT

An electronic auxiliary contact for a contactor has at least one switch element controllable to the conducting and non-conducting states for the purpose of indicating the position of the contact bridging member in the contactor, and a switch-activating element mechanically connected to the contact bridging member and capable of activating the switch element. The switch-activating element incorporates at least one activating element, and the switch-activating element makes no mechanical contact with the switch element. The system provides a reliable method of monitoring the operation of a contactor.

FIELD OF THE INVENTION

The present invention relates to an electronic auxiliary contact for aelectromechanical contactor having a movable slider and at least oneswitch element.

DESCRIPTION OF THE BACKGROUND ART

Conventional contactors employ mechanical auxiliary contacts accordingto FIG. 1 to accomplish different kinds of control and monitoringsignals. The contact block is constructed with the help of a movingcontact element a attached to the contact bridging member, said elementcarrying a portion of the circuit b of the auxiliary contact, saidportion opening and closing the circuit c when said contact element isin either of its home positions. The contacts are sprung to attainlonger mechanical life.

A mechanical contact is suitable for controls performed at conventionalmains voltage levels, but developments in automation technology have setnew demands on the quality of control and monitoring signals. Lowvoltage level and small current as well as precise timing of the signalsare now desirable. Information on the position of a contactor's contactbridging member, for instance, can be signalled by means of mechanicalauxiliary contacts, but this prior-art technique involves problems thatcompromise the reliability of obtained information. Bouncing of contactsat the opening and closing generates signal transients which causejitter in the precise timing of events, and due to the mechanical playof contacts, the timing of the obtained signal has insufficient accuracyfor positional monitoring of the contact bridging member. Moreover,contamination and oxidation of contact surfaces cause malfunctions,particularly at low current and voltage levels.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above-describeddisadvantages and to achieve a novel system for obtaining and utilizinginformation on the positions of a contactor.

The system according to the present invention is based on the idea thatthe actuating element of the auxiliary contact operates withoutmechanically contacting the actual switch body of the auxiliary contact.

More specifically, the auxiliary contact according to the invention ischaracterized by a switch-activating element which incorporates at leastone activating element and makes no mechanical contact with the switchelement.

An electronic auxiliary contact according to the invention offerssubstantial benefits with respect to the conventional technology. Thus,a stable signal is obtained from the position of the contact bridgingmember, contact bouncing transients are avoided and the timing precisionof the positional information is jitter-free. Due to the hermeticprotection of the switch element, the characteristics of the auxiliarycontact are not deteriorated by contamination or oxidation. Theelectronic auxiliary contact has no moving parts thus retaining aconstant timing precision of the positional information even in extendeduse, and isolation problems are relieved by the mechanicallynoncontacting nature of the auxiliary contact switch circuit in respectto the moving pans.

The electronic auxiliary contact can provide reliable positionalinformation for a process control computer also from a circuit operatingat a low supply voltage. The position of the contactor's contactbridging member can be determined with an extremely high precision. Theelectronic auxiliary contact has a simple construction, and byintegrating more electronics to it, possibilities of multiple differentmonitoring function are feasible not ever attainable at a sufficientaccuracy by means of a mechanically operating auxiliary contact. Asingle auxiliary contact block can incorporate one or more positionsensors, thus making it possible to detect various intermediatepositions complementing the conventionally indicated home positions.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is next examined in detail with the help of the attacheddrawings and exemplifying embodiments illustrated therein which aregiven by way of illustration only and thus are not limitative of thepresent invention.

FIG. 1 shows a conventional auxiliary contact in a side view.

FIG. 2 shows a perpective view of an auxiliary contact according to theinvention attached to a contactor.

FIG. 3 shows a sectional view of an embodiment of the auxiliary contactaccording to the invention illustrated in FIG. 2.

FIG. 4 shows an alternative embodiment of an auxiliary contact accordingto the invention in a perspective view.

FIG. 5 shows a block diagram of the signal processing block of anauxiliary contact according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to FIG. 2, the contactor's auxiliary contact block 1 isfastened to the body 2 of the contactor. According to FIG. 3, theauxiliary contact block is comprised of a two-part body block 4 and anoblong slider 3, which is situated inside the body block 4 and isguidedly movable along a groove 5. The slider 3 incorporates a peg 8aligned to one end of the slider perpendicularly to the longitudinalaxis of the slider. The peg 8 is oriented toward the inner side of thecontactor 2. Thus, the auxiliary contact block 1 is fastened by fittingthe peg 8 of the slider 3 to the contact bridging member (not shown) ofthe contactor 2. The slider 3 is moved along the groove 5 exactly by thesame distance as the contact bridging member of the contactor 2 movesbetween its upper and lower home positions. The position of the slider 3is detected by sensors 6 and 6' which are permanently mounted to thebody block 4 of the auxiliary contact block 1, whereby said sensors inthe described embodiment are Hall sensors. Alternatively, the sensorscan be replaced by inductive or capacitive proximity sensors or opticalgap sensors. Magnetically-activated switch contacts are also feasible assensors, but they do not offer as good precision as is attainable withthe other sensor types described above. One auxiliary contact block 1requires two sensors 6 and 6'; one sensor serving for the upper and onefor the lower home position, respectively. The functions of asingle-position mechanical auxiliary contact can be accomplished using asingle sensor alone.

FIG. 3 illustrates a construction having two sensors 6 and 6', wherebysaid construction is capable of replacing a dual-function mechanicallyswitched contact. The sensors 6 act as switch elements and are placed onthe upper surface of the slider 3. Because the sensors 6 and 6' are ofthe Hall sensor type, a small permanent magnet 7 is fastened to theslider 3, whereby the motion of the magnet to coincide with the sensor 6or 6' sets the respective sensor to the conducting state. Thisarrangement makes it possible to adjust the effective stroke of theslider and the activation positions of the sensors by varying the sizeof the magnet 7 and the mutual distance of the Hall sensors 6 and 6'.Because the Hall sensors 6 and 6' will only be activated by a magneticflux aligned perpendicular to their measuring surface, it is possible toselect a suitable sensor/magnet combination and align these elementsappropriately with respect to the flux emitted by the main magneticcircuit of the contactor 2 so that an extremely accurate sensorfunction, free from interference by stray fields, is achieved. Thesignals from the sensors 6 and 6' are taken in a cable 35 to a signalprocessing unit to be described below; thus, an embodiment based on Hallsensors requires a cable with three separate conductors, one for eachHall sensor element. In practice the number of conductors is increasedto the qty. of sensors plus two, because each sensor element needs aseparate signal line complemented with a common ground and supplyvoltage line. Therefore, the number of conductors necessary in thedescribed embodiment is 4.

FIG. 4 shows a corresponding construction suitable for inductive orcapacitive sensors 9. The mechanical basic construction herein issimilar to that described for the embodiment illustrated in FIG. 3, sotwo sensors 9 are also needed in this embodiment. This kind of sensors 9can operate without an external magnetic flux, because they sense theproximity of a metallic vane 10 in front of their sensing surface.Therefore, the slider 3 is provided with small metal plates 10 which arealigned with respect to the sensors 9 so that one plate is coincidentwith the upper position sensor when the contact bridging member is inits upper home position and, correspondingly, the other plate iscoincident with the lower position sensor when the bridging member is inits lower home position.

The mechanical design of the auxiliary contact block can vary fordifferent types of contactors, yet maintaining an identical principle ofoperation.

An electronic auxiliary contact operates at a low supply voltage of 5-48VDC depending on the sensor type used. Hall sensors as well asinductive, capacitive and optical sensors require a separate supplyvoltage line and a dedicated signal line. Maximum allowable load currentfrom the sensor output stages is limited to a few tens of milliamperesat its best, so an electronic power driver stage or relay is necessaryfor controlling voltages or currents at higher levels. In mostapplications the operating environment tends to cause interference withthe measurement, so the output signal from the sensor elements must beprocessed by electronic means in either the sensor block, its immediatevicinity or the automation system.

An application of the electronic auxiliary contact is in contactbridging member position monitoring of contactors. The contactor'selectronic auxiliary contact block having separate sensors for the upperand lower home position sensing is connected to a logic circuit shown inFIG. 5. The logic circuit comprises inputs for a lower-position sensorsignal 30 and an upper-position sensor signal 31, inverting Schmitttriggers 12 and 13, distal low-pass filters 14 and 15, and a positionalinformation processing logic circuitry comprised of three NOR gates 16,17 and 18, and one AND gate 19. The logic circuitry processes inputsignals taken to the inputs 30 and 31 into four differentstate-indicating signals defined as: Signal "Sensors disconnected" 20,signal "Contact bridge driven up" 21, signal "Contact bridge midway" 22and signal "Contact bridge driven down" 23.

In severe operating conditions the input signals will carry superimposedinterference consisting of mains frequency, harmonics or high-frequencytransients caused by frequency converters and other switch-mode powersources. The Schmitt triggers 12 and 13 at the logic inputs filter awaylow-amplitude interference from the input signals irrespective of theirfrequency. The filtration result will be the more effective the widerthe hysteresis of the Schmitt trigger 12 or 13. Further improvement infiltration can be obtained by using a large input voltage swing. Theinputs are taken high by pull-up resistors 32 and 33 for the purpose ofsensing the integrity of sensor connections.

At the second stage the interference components managing to pass theSchmitt triggers 12 and 13 are filtered away by one-bit digital filters14 or 15. The filter 14, 15 can be a median-producing filter or anonlinear low-pass filter that removes transients from the signal.

The logic circuit described above for processing of positionalinformation is implemented for detection of a sensor 6 in the conductingpositions and an activated sensor is in the non-conducting position. Ifneither of the sensors 6 is activated, that is, both of their outputsignals are taken as being low, the contact bridging member isinterpreted to be in a midway position. Signal indicating this state isformed by the AND gate 19. A situation having both sensors 6 activatedis considered impossible, so its occurrence can be interpreted toindicate severed connection to the sensors. Signal indicating this stateis formed by NOR gate 16. When Hall sensors are employed, their outputsignal properties must be considered in the placement of the sensors.Signals "Contact bridge driven up" and "Contact bridge driven down" areformed by NOR gates from the input signals 30 and 31, complemented withthe signal "Sensors disconnected" 20.

Several auxiliary contact blocks 1 can be connected in parallel, whichin larger contactors offers a possibility of detecting contact bridgingmember jamming slantingly that generally is indicative of contactwelding. Information on contact bridging member position can be employedeven in a wider scale for controlling a contactor. For instance, theposition state signals can be utilized to monitor contactor openingduring hold and then to activate necessary functions to re-establishcontactor hold.

Any of the discussed sensor types are suitable for use with theabove-described circuitry provided that they incorporate anopen-collector output stage capable of driving the logic circuitrysensor inputs to a logic zero state.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. An electronic auxiliary contact for a contactor,said auxiliary contact comprising:movable slider mechanically connectedto a contact bridging member for following change of position of thecontact bridging member; at least one switch element cooperating withthe slider for moving between conducting and non-conducting positionsfor indicating the position of the contact bridging member in saidcontactor; and at least one activating element for activating saidswitch element, the at least one activating element being incorporatedin the slider; said slider failing to make mechanical contact with saidat least one switch element.
 2. The auxiliary contact as defined inclaim 1, wherein said activating element is a permanent magnet and saidswitch element is a Hall sensor.
 3. The auxiliary contact as defined inclaim 1, wherein said activating element is a piece of material having ametallic character and said switch element is an inductive sensor. 4.The auxiliary contact as defined in any of the foregoing claims, whereinthe switch outputs a signal triggered by said activating elements, saidauxiliary contact incorporates a signal-processing unit for eliminationof interference and processing of information related to the position ofsaid contact bridging member of said contactor.